Solar batteries have recently become an active area of research, and improvements have been made from various points of view. A solar battery includes a semiconductor circuit, which is formed by a number of solar cells connected in series, formed on a substrate. Each solar cell has a multi-layer structure, in which a photoelectric conversion layer, which is a semiconductor to generate an electric charge when absorbing light, is disposed between a lower electrode (back electrode) and an upper electrode (transparent electrode).
Conventionally, glass substrates have been used as the substrates in the solar batteries. However, it has been considered to use a flexible metal substrate. A solar battery using a metal substrate has possibility of wider applicability than a solar battery using a glass substrate, because of lightweight and flexibility of the substrate. Further, since the metal substrate is tolerable to a high temperature process, it is expected to provide a solar battery with improved photoelectric conversion characteristics and higher efficiency. However, since the semiconductor circuit, which conducts the photoelectric conversion, of the solar battery is disposed on the substrate, it is necessary to provide an insulating layer between the substrate and the semiconductor circuit when the metal substrate is used.
For example, in a case where the substrate is made of an iron-based material, such as stainless steel, it is known to form the insulating layer by coating an oxide of Si or Al through a gas-phase process, such as CVD (Chemical Vapor Deposition), or a liquid-phase process, such as sol-gel method. However, in general, these types of film forming processes tend to produce pin holes and/or cracking, and this is an essential problem for a process which is used to stably form the insulating layer having a large area.
In a case where the metal substrate is made of aluminum (Al), a good insulating coating with good adhesion is provided by forming an anodized coating on the surface of the Al substrate. However, although the anodized coating has good adhesion, its insulation performance is insufficient, and thus still should be improved to be used as the insulating layer of a solar battery, etc.
With respect to the metal substrate for the solar battery described-above, the maximum voltage generated by each solar cell is about 0.65 V, and 100 or more solar cells are connected in series on a single substrate to form a module circuit. Therefore, considering safety and long-term reliability, the insulating layer on the metal substrate needs to have a withstand voltage of not less than 500 V. Further, it is important that the insulating layer has a high resistance value since a leakage current at the insulating layer is a factor of degradation of solar light-electric power conversion efficiency of the solar battery module.
The anodized film, however, typically has poor insulation performance. There are some known examples of improvement of the insulation performance of the anodized film formed on the surface of the Al substrate. Specifically, a method where an insulating layer is additionally formed on the anodized film (see Japanese Unexamined Patent Publication No. 7(1995)-147416), a method where an intermetallic compound in the anodized film is prescribed (see Japanese Unexamined Patent Publication No. 2002-241992), a method where the thickness of a barrier layer (a thin and dense oxide layer present in the vicinity of the interface between the anodized film and Al) is increased through a pore filling method (see Japanese Unexamined Patent Publication No. 2003-330249, and H. Takahashi and M. Nagayama, “Pore-Filling of Porous Anodic Oxide Films on Aluminium”, Journal of the Metal Finishing Society of Japan, Vol. 27, No. 7, pp. 338-343, 1976) have been reported.
Insulation performance factors of the insulating layer include withstand voltage and leakage current, and a withstand voltage as high as possible and a leakage current as small as possible are desirable. Although the above-mentioned prior-art methods improve these insulation performance factors, these methods are not related to solar batteries, and are to improve the insulation performance of anodized films in general. Therefore, a novel method, other than these methods, which can improve the insulation performance of the anodized film of the Al substrate of the solar battery is desired.